Apparatus for correcting electron beam deflection

ABSTRACT

AN APPARATUS FOR CORRECTING ELECTRON BEAM DEFLECTION IN ELECTRON MICROSCOPES CAUSED BY ASYMMETRICAL LEAKAGE FLUX COMPRISING A SMALL MAGNETIC CIRCUIT WHICH GENERATES A MAGNETIC FIELD OPPOSITE TO THE DEFLECTING COMPONENT OF SAID LEAKAGE FLUX, THE MAGNETIC CIRCUIT BEING SMALL AS COMPARED WITH THE MAGNETIC CIRCUITS OF THE ELECTRON LENS, BEING MADE OF THE SAME MATERIAL AS THE LENSES AND BEING ENERGIZED BY A CURRENT PROPORTIONAL TO THE CURRENT ENERGIZING THE MAGNETIC CIRCUITS OF THE LENSES.

United States Patent Inventor Appl. No.

Filed Patented Assignee Priority Takashi Yanakn Tokyo. Japan Apr. 22, 1969 June 28, I97 1 Nlhon Denshi Xabushiki Kaisha Tokyo, Japan Apr. 26, 1968 Japan APPARATUS FOR CORRECTING ELECTRON BEAM DEFLECTION 9 Claims, 9 Drawing Figs.

U.S. CI 315/18, 315/31 {51] InLCl ..H0lj29/70 [50] FieldofSearch 3l5/18,27

Primary ExaminerRodney D. Bennett, Sr. Assistant ExaminerJoseph G. Baxter Attorney-Webb, Burden, Robinson and Webb PATENTEU M2 8 I97i SHEET 1 BF 2 m "HEW/f4 INVENTOR.

TAKAS HI YAN AKA PATENTEB JUH28 l8?! 3,588,588

SHEET 2 OF 2 [XI an 4.

INVENTOR.

TAKASHI YANAKA APPARATUS FOR CORRECTING ELECTRON BEAM DEF LECTION The present invention relates to corpuscular ray apparatus, such as electron microscopes using magnetic-type electron lenses and, more particularly, to corpuscular ray apparatus in which the applied electron beam deflected by leakage flux emanating from the electron lenses is always corrected.

It is well-known among those skilled in the art that the conventional electron microscope employs magnetic lenses in order to obtain high resolving power. By utilizing a lens producing its own magnetic field, the focal length of the lens can be varied greatly by electrically varying the intensity of the field. Accordingly, focusing, illumination and magnification adjustments which are necessary for satisfactory image observation can be executed with comparative ease. Simultaneously. however, the flux leakage increases in proportion to the increase in the intensity ofthc magnetic field; that is, by increasing the lens current, the amount of flux leakage is correspondingly increased. Moreover, the distribution of the flux leakage changes; that is, the distribution is modulated because of the heterogeneity of the ferromagnetic material from which the lens is made. Generally, the heterogeneity is caused by mechanical stress, impurity of material, forging faults such as blowholes, etc., and microscopic errors in the uniformity of the gaps where each member of the magnetic circuit is joined. Therefore, insofar as electron lenses are constructed of ferromagnetic materials, the elimination of magnetic flux leakage is impossible.

Furthermore, the flux leakage emanating from each electron lens coexists within the specimen chamber located between the condenser lens and the objective lens. Since the conventional specimen chamber is designed to incorporate various specimen devices, the composition of the chamber is inevitably asymmetrical, with the result that the flux leakage becomes nonrotationally symmetrical. As a consequence, the electron beam is deflected by the collective effect of the total leakage fluxes, making it extremely difficult to align the center of the said electron beam and the optical axis. Additionally, since nonaxial aberrations such as astigmatism, coma aberrations, etc., are increased, the resolution of the image is adversely effected.

Moreover, in microbeam diffraction, recently attempted by microscopists, the intermediate lens current must be changed when converting from an electron microscopic image to a diffraction pattern. Simultaneously, with the current change, the intensity of the flux leakage in the specimen chamber varies resulting in beam deflection which leads to a shift in the irradiated portion of the specimen surface during the current change. As a result, the electron microscopic image goes not correspond with the diffraction pattern.

Accordingly, I have invented an apparatus for correcting the deflection of an applied electron beam. Furthermore, my invention provides an improved apparatus for correcting the deflection of an applied electron beam in which the beam deflected by an asymmetric magnetic field formed by flux leakage emanating from the electron lenses themselves is always corrected, with the result that the position at which the beam irradiates upon the specimen surface remains constant.

Generally, my invention provides a novel magnetic circuit, small as compared with the magnetic circuit of the electron lenses and made of the same material. The current energizing the small magnetic circuit is designed to change proportionally with the current energizing the electron lenses. In accordance with this arrangement, the magnetic field componcnt for deflecting the electron beam emanates from the small magnetic circuit.

In the accompanying drawings, l have shown preferred embodiments of my invention in which:

FIG. I is a schematic view of an electron microscope showing the position of my present invention therein;

FIG. 2 is an enlarged cross-sectional view of the objective lens and the correcting element;

FIG. 3 shows an enlargement of the correcting element shown in FIG. 2;

FIGS. 4a and b illustrate magnetic field distribution in the vicinity of the pole pieces shown in FIG. 3; and,

FIGS. 5 to 8 show other embodiments ofthis invention.

In the electron microscope shown in FIG. 1, column 1 comprises a chamber 2 in which an electron gun 3 and an anode 4 are housed. A condenser lens system 5, a specimen chamber 6 housing a specimen 7, an objective lens 8, an intermediate lens 9, a projector lens 10 and a viewing chamber 11 complete with a viewing window I2 and a fluorescent screen 13 are aligned with respect to the electron gun. A correcting element 14 is set within specimen chamber 6, and serves to correct the applied electron beam 15 deflected by the various leakage fluxes coexisting within the specimen chamber.

FIG. 2 shows the relationship between the applied electron beam, the correcting element, the objective lens and the electric power supply. The objective lens 8 comprises an upper ole piece 16, a lower pole piece I7, a yoke 18 and a coil I9. The current energizing lens 8 is supplied from an electric power supply 20 through current adjusting circuit 2]. The current is proportional to the current energizing the correcting element 14; that is, the energizing current applied to correcting element I4 varies with the change in energizing current applied to the objective lens.

For the sake of illustration, the magnetic flux density B of yoke 18 is assumed cquipollent. Hence, the leakage flux field of the objective lens may be defined by a series of concentric or generally parallel curves representing equipotential surfaces. These conccntric curves K,, K K;,... K,, (not shown in the drawings) pass through corresponding points K,, K K;,... K, marked on both the right and left sides of the objective lens (see FIG. 2). When considering concentric curves K,, K K -K,, (having uniform intervals h in the same direction as the magnetic flux indicated by the arrows), each static magnetic potential of the concentric curves, where the potentials are measured from the K, potential, is given as follows:

where 1." represents the permeability of the yoke.

Thus, the leakage magnetic field is readily determined in ac cordance with the distribution of the magnetic potential along the outer surface of the objective lens. In general, the magnetic flux density B is proportional to the lens energizing current I. But as is well-known, magnetic field intensity H has hysteresis with respect to B. As a result, the various magnetic field intensities correspond to one constant value B or I. More explicitly, the magnetic field intensity H changes in accordance with the material constituting the magnetic circuit and/or during the process of the magnetic flux density reaching B (which increases in proportion to the increase in lens energizing current). Furthermore, the spatial distribution of the magnetic field intensity and the pattern of the lines of magnetic force about the leakage magnetic field are determined simply by the magnetic field intensity along the lens outer surface. Consequently, the intensity of the asymmetrical magnetic field due to the leakage flux is proportional to the magnetic field intensity H. As a result, the applied electron beam 15 is deflected proportionally to the magnetic field intensity.

In FIG. 2, electron beam I5 is assumed deflected at the position of line 22; the beam, deflected by the leakage magnetic field, is corrected by correcting element 14.

A small magnetic circuit I40, which is small (e.g., one-fifth to one-twentieth) compared with the size of the objective lens. is included in the correcting element I4 (See FIG. 3). The circuit comprises a yoke 23, a gap 24 and a coil 25. The yoke has a constant magnetic flux density B. The leakage magnetic field may be represented by concentric curves C,, C C C,, (not shown in the drawings) passing through points C C C C, on both the right and left sides of the yoke. Each curve has a constant static magnetic potential. The energizing current applied to this circuit is designed to vary with the change in current applied to the objective lens. Furthermore, the small magnetic circuit is constructed of a material similar to that of the objective lens. Thus, both the magnetic circuit of the objective lens 8 and that of small magnetic circuit 140, which preserves the condition of the complete demagnetization, are simultaneously energized by electric source 20 shown in FIG. 2. However, to make the magnetic field intensity H, which corresponds to the magnetic flux density B of the objective lens yoke, equal to the magnetic field intensity H of the small magnetic circuit, the magnetic flux densities of both circuits are preset accordingly. This is accomplished by adjusting the ratio of the current intensity fed to the objective lens to the current intensity fed to the circuit element 14 and/or the gap 24 ofthe magnetic circuit Mu. In the embodiment shown in FIG. 2, a pattern of the lines of magnetic force ha ing the same intensity as those of the leakage magnetic field emanating from the objective lens is generated in the vicinity ofthe small magnetic circuit. The magnetic potential differences at each point (C to C,,) on the surface of the small magnetic circuit change in proportion to the magnetic field intensity H emanating from the objective lens.

On the other hand, since the yokes 26a and 26b are connected magnetically with the two points (C and C shown in FIG. 3) on magnetic circuit 14a and with magnetic pole pieces 27a and 27b, respectively, the static magnetic potential between C and C is applied to the upper and lower pole pieces 27a and 27b. As a result, the intensity of the deflected magnetic field between the upper and lower pole pieces is proportional to the magnetic potential difference between C;, and C Moreover, the magnetic potential difference is proportional to the intensity of the leakage magnetic field emanating from the small magnetic circuit surface, and the intensity of the leakage magnetic field coincides with the intensity of the leakage magnetic field emanating from the objective lens surface. Accordingly, the intensity of the deflected magnetic field is in proportion to the intensity of the leakage magnetic field emanating from the objective lens surface.

In order to enable the amount of eccentricity between the upper and lower pole pieces 27a and 27b to be varied, lower pole piece 27b is movably connected to yoke 26b.

The relationship between pole piece eccentricity and the deflecting component of the magnetic field is shown in FIG. 4. In FIG. 4a, the distribution of the magnetic field is symmetrical, with respect to the central axis of both pole pieces; that is, the pole pieces are concentric. Under this condition, magnetic field intensity E1 generates about the axis and the electron beam irradiated along the axis maintains a straight path. Conversely, in FIG. 4b, since the pole pieces are eccentric, a deflection component B the force of which is determined in addition to the amount of eccentricity by such factors as magnetomotive force and gap size, is generated about the axis perpendicularly with the result that the distribution of the magnetic field becomes asymmetrical, and the applied electron beam is deflected by component B The amount and direction of the eccentricity can be adjusted to irradiate the electron beam onto the center of the specimen surface by means of a suitable mechanism (not shown) linked to lower pole piece 27b. Provided this adjustment can be affected at one try, 8,, will change proportionally to the deflected component of the leakage flux emanating from the objective lens because the energizing current applied to the small magnetic circuit is in proportion to the current variation applied to the objective lens. Accordingly, the applied electron beam is always corrected by deflected component B,, so that the beam always irradiates on the center of the specimen surface.

The design of the small magnetic circuit 14a shown in FIG. is different from that of the objective lens. In this case, two coils 29a and 2% are wound around core 28 which has a gap 30 between which an external magnetic field is produced. In this embodiment, the material of tore 23 comprising a mag netic circuit, and the energizing current applied to both magnetic circuits are the same as for the aforesaid embodiment. By suitably varying gap 30, a magnetomotive force generated in proportion to the intensity of the leakage magnetic field emanating from the objective lens can always be applied to the gap between the upper and lower pole pieces 27a and 27b. Consequently, this embodiment produces the same result as the embodiment shown in FIG. 3.

In FIG. 6, correcting element 14 (a small magnetic circuit in itself) is inserted and set into the specimen chamber 31 at right-angles to the central axis. The element is made of a material similar to or the same as that of the objective lens, and includes a core 33 onto which an energizing coil 32 is wound and an external cylinder 34 positioned. Both core 33 and cylinder 34 are mounted to move relatively and slidably. In accordance with this arrangement, it is possible to adjust the distance of gap 35 provided between the top of core 33 and the bottom of external cylinder 34 and the correcting element I4 is constituted so as to move freely and slidably about the specimen chamber 31. Thus, the intensity of the deflected magnetic field (compensating component) can be adjusted by changing the distance (I) of the part projecting within specimen chamber 31. The leakage magnetic field (deflected magnetic field) emanating from the correcting element 14, is, therefore, utilized to correct the electron beam deflection. In order to design a device on which the intensity of the leakage magnetic field emanating from the external surface of external cylinder 34 base changes proportionally to the magnetic field intensity of the objective lens, gap 35 is adjusted, and/or the thickness of each member comprising external cylinder 34 is predetermined. Furthermore, to adjust the electron beam deflection, the correcting element (small magnetic circuit) must be set so as to rotate about the optical axis. Alternatively, a plurality of correcting elements are necessary. In the latter case, the deflected electron beam is corrected by the resultant magnetic field formed by the leakage magnetic field emanating from each correcting element.

The correcting element shown in FIG. 7 comprises a small magnetic circuit 140, a ferromagnetic plate 36, a nonmagnetic case 37 and adjusting rods 38a and 38b. Magnetic circuit comprises a yoke 39 on which a coil 4] is wound, and a cylindrical yoke 40 which partially surrounds the yoke and coil. Gap 42 is adjusted by manipulating cylindrical yoke 40. Moreover, the correcting element is designed to shift the assembled parts enclosed in the nonmagnetic case 37 by manipulating adjusting rod 38aand 38b so as to push against flange 43. Accordingly, it is possible to shift the aperture axis of yoke 39 in either direction so that a state of eccentricity exists between the aperture axis and the aperture axis of the ferromagnetic plate 36. Moreover, since the magnetomotive force between A and B is applied to the gap between the ferromagnetic plate 36 and the upper part of yoke 44, which forms the other magnetic pole, the correcting element is positioned in the specimen chamber so that the axis of the ferromagnetic plate 36 coincides with the axis of the condenser or objective lens.

Furthermore, the small magnetic circuit is made of the same or a similar material to the objective lens, and the plate between A and B is much thinner than the other part of the yoke 39. By manipulating cylindrical yoke 40, gap 42 is adjusted to make the magnetic flux densities of both the small magnetic circuit and the objective lens coincide. The magnetomotive force between A and B is applied without loss to the gap between the ferromagne ic plate 36 and the upper part (magnetic pole) of yoke 44 because the ferromagnetic plate 36 is sufficiently thick. Furthermore, in order to eliminate magnetic reluctance, flange 43 is firmly secured to ferromagnetic plate 36. As a result, the magnetomotive force applied to the gap is proportional to the intensity of the leakage magnetic field emanating from the objective lens surface, and is proportional to the intensity of the deflected magnetic field in the specimen chamber. By manipulating adjusting rods 38a and 38b, it is possible to correct the deflected electron beam.

FIG. 8 shows a duplicated embodiment of the device shown in FIG. 7. Here, the correcting element comprises two small magnetic circuits 14a and 14b having two yokes 39a and 39b, two cylindrical yokes 40a and 40b, two coils 4la and 4lb, two ferromagnetic plates 36a and 36b, and four adjusting rods 38a,

38b, 38c and 38d. The two magnetic circuits I40 and 14b are movable. One of the correcting elements connects with the objective lens. and the other end with one of the condenser lens, the intermediate lens and the projector lens. Accordingly, if the electron beam is deflected by the leakage magnetic field emanating from the various lenses constituting the instrument, it is possible to effect complete correction by setting each correcting element according to the said lenses.

lclaim: I. An apparatus for correcting an electron beam deflection due to leakage magnetic fields emanating from magnetictype electron lenses, said apparatus comprising:

A. a magnetic circuit small as compared with the magnetic circuit of the electron lens and made of substantially the same material as said lens; B. means for applying an energizing current to the small magnetic circuit which is proportional to the energizing current applied to the electron lens; C. means connected to two points on the small magnetic circuit for producing a correcting magnetic field said means being positioned to form the correcting field on the optical axis and said field being proportional to the magnetomotive force between the two connection points on the small magnetic circuit.

2. An apparatus as set forth in claim 1 in which the means for producing the correcting magnetic field comprise at least two pole pieces positioned on the optical axis of the lens.

3. An apparatus as set forth in claim 1 wherein said magnetic circuit comprises a yoke having a gap and an internal coil, said yoke having the same magnetic flux density as the yoke of concerned electron lens and said coil being connected to the means for applying energizing current.

4. An apparatus as set forth in claim 2 wherein at least one of said pole pieces is movable to adjust the eccentricity between said pole pieces.

5. An apparatus as set forth in claim l wherein said magnetic circuit comprises a yoke having a pair of external coils wound around it and a gap in which an external magnetic field is produced.

6. An apparatus for correcting an electron beam deflection due to leakage magnetic fields emanating from magnetic-type electron lenses, said apparatus comprising:

A. a core;

B. an energizing coil wound around said core;

C. an external cylinder enclosing said core and winding provided with a variable gap between the top of the core and bottom of said cylinder;

D. the core and cylinder extending into a column of a corpuscular ray apparatus and being mounted for free movement therein; and,

E. means for applying an energizing current to said coil proportional to the energizing current supplied to the electron lens, the leakage flux from the cylinder correcting deflection of the electron beam.

7. An apparatus as set forth in claim 1 in which the small magnetic circuit is positioned coaxially with respect to the electron lens, and a ferromagnetic plate having an aperture therein aligned with the optical axis of the lens slidably contacts a portion of the small magnetic circuit whereby the magnetomotive force between the one portion of the small magnetic circuit and an other portion on the small magnetic circuit is applied to a gap between the plate and the other portion on the small magnetic circuit.

8. An apparatus for correcting an electron beam deflection due to leakage magnetic fields emanating from magnetic-type electron lenses, said apparatus comprising:

A. a circular ferromagnetic plate having an aperture therein aligned with the optical axis of said lenses;

B. a cylindrical, movable yoke with an aperture, said yoke positioned adjacent to said plate and including i. an energizing coil wound about said yoke and ii. a flange extending slidably contacting the plate;

C. a housing for said yoke, flange and COll mounted to said yoke and having an aperture aligned with the plate aperture;

D. means for adjusting the yoke to vary the alignment of the yoke aperture with respect to the apertures of said housing and plate;

E. means for applying energizing current to said coil proportional to the energizing current applied to the electron lenses; and,

F. the magnetomotive force between the flange and an other portion of the yoke being applied to a gap between the plate and said other portion of the yoke.

9. An apparatus as set forth in claim 8 wherein the correcting apparatus includes two yokes mounted back to back, each having separate adjusting means with respect to the alignment of their respective apertures; a second ferromagnetic plate with an aperture aligned with the aperture of said other plate, and a second partial yoke for adjusting the gapbetween said second plate and said second flange on said secor d yoke. 

